In about 1969, when shoulder replacements using the Neer type of prosthesis were coming into widespread use, the humeral head was an integral part of the prosthesis. As shoulder replacement became more widely used, modular systems were introduced. Humeral heads of different sizes were available, with plastic trial shapes, to ensure that the best size was chosen.
Available information reports that normal movements of the shoulder showed a range of 90.degree. internal rotation to 90.degree. of external rotation, i.e. a total range of 180.degree.. Accordingly, known humeral head prostheses, such as those described above, were based on a hemisphere. This appeared to be close to the anatomical shape, and was therefore entirely suitable. However, the long head of the biceps muscle which passes over the anterior aspect of the shoulder impinges against the angular shape of a hemispherical humeral head. This causes pain and fraying of the tendon.